Semiconductor devices often have one or more layers of a metal deposited onto the device during the fabrication of the device. These deposited layers may be used to form interconnects to electrically couple various portions of the device. Layers of insulative material may likewise be deposited onto the device during fabrication.
A number of techniques have been developed to deposit layers of material including chemical vapor deposition (CVD) and physical vapor deposition (PVD), the latter often referred to as "sputtering." In a typical sputtering chamber, a target formed of the desired deposition material is positioned in the vicinity of the substrate onto which the target material is to be deposited. A plasma is created adjacent to the target which is typically biased so as to attract the plasma ions to the target. Ions attracted to this target can impact the target with sufficient force to dislodge i.e. "sputter," material from the surface of the target.
Because the target material tends to be sputtered in all directions of the interior of the chamber and not just onto the device substrate, many sputtering chambers have a shield which encircles the chamber space between the target and the workpiece. The shield confines the sputtered material to minimize deposition of sputtered material on the interior walls of the chamber. This shield is typically at electrical ground whereas the target is usually biased (typically negatively biased) to attract the sputtering ions. Hence, it is desirable in such chambers to space the target from the shield to prevent an electrical short between the target and the shield.
On the other hand, the plasma created adjacent to the target can itself provide a conductive path which can short the sides of the target to that portion of the shield adjacent to the sides of the target. To avoid such a plasma induced short, it has been found that the formation of a plasma between the target sides and the adjacent shield may be inhibited by narrowing the space between the target and the shield to not exceed a predetermined maximum, which may be referred to as the "dark space" gap.
Thus, it is appreciated that achieving good alignment between the shield and the target is important in many applications. If the target is installed either too close or too far from the shield, the sputtering chamber may not operate properly. In addition to undesirable arcing between the shield and the target, the quality of the deposition may be adversely affected as well. Such improper alignment may not be detected until after valuable wafers have been ruined and the source of the problem properly traced to target misalignment.
The size of the desired darkspace gap between the target and the shield will vary, depending upon the particular application. In many applications, the maximum desired darkspace is on the order of millimeters. On the other hand, targets are usually quite large and heavy, varying in size from 200-250 mm in diameter or more. Thus, attaining proper alignment during installation of the target can be difficult to achieve. Because the shield is usually even larger in size, a smaller shield, referred to as a "darkspace" shield has been used in combination with the main shield. While use of a separate darkspace shield adjacent to the target has facilitated aligning the target to the shield, consistently achieving proper alignment has continued to be a problem in many applications. Hence, there is a need to reliably detect target misalignment before device production is resumed following installation of the target.